Review of asphalt binder grading systems for hot mix asphalt ...

RESEARCH ARTICLE

Review of asphalt binder grading systems for hot mix asphalt

pavements in Sri Lanka

* Corresponding author ([email protected])

W.K. Mampearachchi *, G.S. Mihirani, B.W.P. Binduhewa and G.D.D. Lalithya

Department of Civil Engineering, Faculty of Engineering, University of Moratuwa, Moratuwa.

J.Natn.Sci.Foundation Sri Lanka 2012 40(4): 311-320

Abstract: There are three main asphalt binder classification

systems; penetration, viscosity and Superpave. The penetration

grade 60 –70 binder is used for road constructions in Sri

Lanka. The objective of this study was to review the existing

asphalt binder grading systems and find the most suitable

grading system for Sri Lanka. Performance of the road sections

constructed with different penetration grade binders were

examined and it revealed that the binder grade significantly

affects the performance.

A survey was conducted among the professionals

involved in the road construction industry to evaluate the

current practices. Laboratory test results of the University of

Moratuwa and test reports collected from road contractors and

the Research and Development (R&D) Division of the Road

Development Authority (RDA) were investigated. The test

results revealed that binders used in the industry are highly

temperature susceptible, though the binder has complied with

the standards.

Data collected from the Meteorological Department for the

last 20 year period were analyzed to estimate the maximum

and minimum pavement temperatures, which are required

for Superpave binder grading. An algorithm validated for

Sri Lankan conditions was used to estimate the pavement

temperature from ambient temperature. The requirement of

different binder grades for different zones has been identified

and a GIS map was developed to select the suitable binder

grades for specific road sections in Sri Lanka. PG 58-16 can

be recommended for majority of roads while PG 52-10 can be

recommended for the regions in the central hilly area of the

country where low temperature prevails. Similarly, AC 30 and

AC 20 can be specified for high and low temperature zones,

respectively.

Keywords: Asphalt, binder grading, penetration, Superpave,

viscosity.

INTRODUCTION

The durability and the long-term satisfactory performance

of pavements (road carriageways) are influenced and

affected to a great extent by the pavement ingredients

(materials) and their inherent properties. It is very

pertinent to consider properties of the bituminous binders

and the bitumen content as a quality control measure

in a bituminous construction. The construction sector

should be interested in using the right type of bitumen

for obtaining durable pavements (Nagabhushana, 2009).

Penetration grading system is the first standard

method adopted by the American Society for Testing

and Materials (ASTM) Committee D04 on road and

paving materials in 1918. The viscosity grading system

was introduced in 1960. Both methods are empirical

methods, based on past experiences and observations.

The latest grading system, the Superpave grading system,

was introduced in 1998. Superpave grading system was

developed as part of the Superpave research effort to

introduce more accurate and fully characterized asphalt

binders for use in hot mix asphalt (HMA) pavements.

It was based on the idea that asphalt binder properties

should be related to the conditions under which it is

used. For asphalt binders, this involves expected climatic

conditions and ageing considerations (Adedimila &

Olutaiwo, 2003).

The penetration grading system is currently used

in Sri Lanka while many of the developed countries

have adopted to the latest Superpave grading system.

Many researches are being conducted on Suprepave

technology. India has developed a grading system based

on viscosity. Hence, reviewing of asphalt binder grading

Revised: 28 May 2012 ; Accepted: 18 July 2012

312 W.K. Mampearachchi et al.

December 2012 Journal of the National Science Foundation of Sri Lanka 40 (4)

systems and identifying the improvements necessary for

the existing limitations and the restrains in the country

are important.

The overall objective of this study was, to review the

asphalt binder grading systems in line with Sri Lankan

environmental conditions. The following tasks were

carried out to achieve the objectives of this study.

Evaluation of the effect of binder grade on pavement 1.

distresses

Analysis of asphalt binders used in the industry with 2.

the evaluation of

temperature susceptibility (experimentally and †

theoretically)

the properties †

mixing and laying temperatures†

current industry practices †

Selection of suitable asphalt binder grades for roads 3.

in different climatic zones of Sri Lanka and develop

GIS maps

LITERATURE REVIEW

Binder testing studies go back to 1888, when H.C.

Bowen invented the Bowen penetration machine

(Welborn & Halstead, 1974). After several modifications,

the penetration equipment became the standard for

establishing the consistency of asphalt at 25 oC by

1910. In 1918 the Bureau of Public Road, USA and

ASTM introduced the penetration grading system. By

1931 the American Association of State Highway and

Transportation Officials (AASHTO) published the

standard specification for grade asphalt on penetration.

The next major change in asphalt grading specification

came with the introduction of the viscosity grading system

in early 1960s. Both ASTM and AASHTO adopted

the viscosity grading system and provided grading

specifications by measuring the viscosity at 60 oC.

ASTM D946 specified five binder grades based upon

penetration at 25 oC. The greater the penetration, the

softer the binder. In the case of viscosity grading,

viscosity at 60 oC (close to maximum pavement

temperature) is specified. The specifications also require

a minimum viscosity to be measured at 135 oC to reduce

the potential of the tender mix at the time of compaction.

ASTM D3381 specifies six binder grades based on the

viscosity measured at 60 oC. Table 1 provides the standard

penetration and viscosity grades.

Generally, softer binder grades are used in the cold

climates to resist cracking potential and harder binders

are used for warmer climates to resist rutting potential.

The viscosity grading system based on a fundamental

property is considered as a step forward in specifying the

binder as compared to penetration grading. It requires the

binder to be tested at 60 oC and 135 oC, which correspond

to the typical maximum pavement temperature and the

temperature at the time of mix production and placement

in the field, respectively. Viscosity specification at

60 oC helps in minimizing the rutting potential, whereas,

viscosity at 135 oC helps to minimize the potential for

tender mixes during the paving operation. Inspite of all

these added benefits, it fails to characterize the binder at

low temperatures to minimize the potential of thermal

cracking and pavement performance prediction.

The Superpave system is unique in that asphalt

binder is specified on the basis of the maximum and

minimum pavement temperatures in which the binder is

expected to serve. The requirements of the mechanical

properties remain the same, while the temperature at

which the asphalt binders achieve the physical properties

corresponds to the minimum and maximum temperatures

of the pavement. For example, high temperature requires

the binder to have G*/sin δ to be at least 1.0 kPa for

unaged condition (G* is the shear modulus in kPa and

δ is the phase angle). The value of 1.0 kPa remains

constant but the temperature at which this value has

to be achieved depends upon the maximum pavement

temperature. Another important feature of Superpave is

that mechanical properties are measured on the asphalt

binders of three conditions: unaged, short-term aged

and long-term aged. The short and long-term ageing is

simulated in the laboratory using the rolling thin film oven

(RTFO) and pressure ageing vessel (PAV), respectively.

The required mechanical properties under the three

ageing conditions, both for high and low temperatures

are specified in the Superpave specifications.

Table 2 presents the typical binder grades as specified

in the Superpave specifications. High temperature of

Penetration grading Viscosity grading

Grade Penetration Grade Viscosity at

in 0.1 mm 60 °C, Poise

Pen 40/50 40 − 50 AC − 40 4000 ± 800

Pen 60/70 60 − 70 AC − 30 3000 ± 600

Pen 85/100 85 − 100 AC − 20 2000 ± 400

Pen 120/150 120 − 150 AC − 10 1000 ± 200

Pen 200/300 200 − 300 AC − 5 500 ± 100

AC − 2.5 250 ± 50

Table 1: Penetration and viscosity grading system

Review of asphalt binder grades 313

Journal of the National Science Foundation of Sri Lanka 40 (4) December 2012

LTPP programme for high and low pavement temperature

predictions.

Tpav,h

= Tair

- 0.00618 Lat2 + 0.2289 Lat +

(42.4) (0.9545) - 17.78+z.σair

...(1)

Tpav,h

= 54.32 + 0.78 Tair

- 0.0025 Lat2 -

15.14 log10

(d + 25) + z (9 + 0.61σ2

air)1/2 ...(2)

Tpav,h

= High AC pavement temperature at 20 mm

from surface, oC

Tpav,h,d

= High AC pavement temperature at depth d

from surface, oC

T air

= High 7-day mean air temperature, oC

Lat = Latitude of the section, degrees

d = Pavement depth, mm

σair

= Standard deviation of the 7-day maximum air

temperature, oC

Z = Standard normal dist. table, z = 2.055 for

98 % reliability, and z = 0.0 for 50 %

reliability

The SHRP high temperature model was developed

from the results of theoretical heat transfer modelling

(Huber, 1994). Based upon the data collected from

several sites throughout the U.S., a regression model

was then developed for prediction of high pavement

temperature as a function of depth. Superpave defines the

high pavement design temperature at a depth of 20 mm

below the pavement surface. Equation 1 represents the

model developed under the SHRP programme, whereas

equation 2 is the LTPP model.

SHRP low temperature models considers the low air

temperature as the design low pavement temperature

(Huber, 1994). The low pavement design temperature at

the pavement surface is the same as the 1-day minimum

temperature, since the air temperature is the same as the

pavement surface temperature. This can be mathematically

represented by the following relationship.

Tpav,l

= Tair

- 0.051xd + 0.000063x d 2 - z.σair

...(3)

The LTPP low pavement temperature at the surface is

presented in equation 4 below.

Tpav,l

= -1.56 + 0.72Tair

- 0.004Lat 2 +

6.26 log10

(d + 25) - z (4.4 + 0.52σ2

air)2 ...(4)

Where Tpav,l = low AC pavement temperature in oC

High temperature Low temperature

grade, °C grade, °C

PG 46 34, 40, 46

PG 52 10, 16, 22, 28, 34, 40, 46

PG 58 16, 22, 28, 34, 40

PG 64 10, 16, 22, 28, 34, 40

PG 70 10, 16, 22, 28, 34, 40

PG 76 10, 16, 22, 28, 34

PG 82 10, 16, 22, 28, 34, 40

Table 2: Superpave binder grades

46 oC has corresponding low temperatures of - 34,

- 40 and - 46 oC, resulting in PG 46-34, PG 46-40 and

PG 46 - 46, respectively. The temperatures given in

Table 2 correspond to the pavement temperatures

and can be estimated from the air temperature data

collected over the years. Superpave defines the high

and low temperatures by 7-day average maximum air

and l-day minimum air temperature. The 7-day average

maximum temperature is defined as the average highest

air temperature for a period of 7 consecutive days within

a given year. The 1-day minimum temperature is defined

as the lowest air temperature recorded in a given year.

The data are collected over multiple years and the design

high and low pavement temperature values are then

estimated using the average and standard deviations of

the data collected for a desired reliability level (Roberts

et al., 1996, Adedimila & Olutaiwo, 2003).

Pavement temperature models

Several research efforts have been made to relate the air

temperature to the pavement temperature. Regression

equations along with mathematical heat flow theories

have been used for establishing the correlation. Among

these, models for the prediction of high and low

pavement temperatures based upon the air temperature

data were established during the strategic highway

research programme (SHRP). However, later SHRP

established the long term pavement performance (LTPP)

programme to support a broad range of pavement

performance analysis leading to improved engineering

tools to design, construct, and manage pavements. In

this regard, the seasonal monitoring programme (SMP),

a task of LTPP evaluated the effects of temperature

variations on performance and validated the available

models (Mohseni, 1998; Diefenderfer et al., 2002). This

resulted in a new set of pavement temperature prediction

models for the high and low temperature grades. Given

below are the models developed under the SHRP and



In the above equations (equation 1 to 4), the factor “z

and air” is included to introduce the equations 5, 6 and 7

that has been successfully used by a research team at the

University of Balamand, Lebanon (Khalil et al., 2009).

Ts(Max)

= Tair(Max)

- 0.00618 · latitude2 +

0.2289 · latitude + 24.4 ...(5)

Td(Max)

= (TS(Max)

+17.8 )(1-2.48 × 10-3d + 1.085 ×

10-5d2 - 20441 × 10-8d3) - 17.8 ...(6)

TS(Min)

= 0.859Tair(Min)

+ 1.7 ...(7)

Where;

Tair (Max)

= 7 – day average maximum air temperature

Tair (Min)

= 1 – day average minimum air temperature

TS (Max)

= Maximum surface temperature

Td (Max)

= Maximum temperature at a depth of d (d is

20 mm for the Superpave binder selection)

TS (Min)

= Minimum surface temperature

(Khalil et al., 2009)

METHODOLOGY

Questionnaire survey

The questionnaire was mainly focused on obtaining

industry practices on asphalt binders grading systems,

binder testing and mixing and laying temperatures. The

survey was conducted among those who are involved in

the highway industry such as project managers, material

engineers, site engineers and laboratory managers.

Distress survey

A case study on the Rathnapura - Bandarawela road

was carried out to identify the binder related distresses

since two types of binder have been used in the road

construction. Four locations under different climatic

conditions (Figure 1 and Table 3) were studied and all

the pavement distresses were noted with their severity.

Pavement performances were evaluated by comparing

the observations of the distresses with the binder grade

used.

Asphalt binder properties

Temperature susceptibility

Asphalt binder is a thermoplastic material and its

consistency changes with the temperature. Temperature

susceptibility is the rate at which the consistency of an

asphalt binder changes with temperature and is a very

important parameter of asphalt cement (Khalil et al.,

2009).

Experimental : Penetration test was conducted at

different temperatures to check the temperature

susceptibility of the binder samples and the penetration

vs. temperature graph was drawn.

Theoretical : The penetration index can be used to

estimate the temperature susceptibility. Penetration

index of each sample was calculated using penetration

and softening point data available in the test reports

and compared with the allowable ranges (Roberts

et al., 1996).

Test report analysis

Bitumen test reports were collected from the R&D section

of the Road Development Authority, Maga Neguma

emulsion plant and contractors. The softening point and

penetration values of test reports were compared with

the allowable ranges for the respective bitumen type

(Roberts et al., 1996).

Data on mixing temperature of asphalt cement at

plant, and laying temperature at site were collected from

contractors in the road sector. The mixing and compaction

temperatures were analysed.

Selection of binder grades

Based on the literature, suitable binder grades were

selected for different areas where meteorological data

were available using all three grading systems. Air

temperature values collected from the Meteorological

Department were used for the analysis. Since Superpave

grading system is based on pavement temperature, the

Superpave prediction algorithms were used to convert

the air temperatures into pavement temperatures (Asphalt

Institute, 2003). The Superpave prediction algorithms

were verified with the Sri Lankan climatic conditions.

Figures 2 and 3 show the thermocouple locations of the

two selected roads and installations of thermocouples for

validation of algorithms.



DATA ANALYSIS AND RESULTS

Pavement performance analysis

Figures 4(a) – 4(d) show the critical distresses identified

in the survey. Summary of the observed pavement

distresses and the binder grade used in these locations

are shown in Table 4. Softer bitumen, 80/100 and harder

bitumen, 60/70 have been used for road sections in the

hot climatic zone and the cold climatic zone, respectively.

Figure 2: Experiment in local roads

Figure 1: Location map

Location Name Binder Distress

type Type Severity

1 Pelmadulla 80/100 Bleeding High

Shoving High

2 Allepola 80/100 Bleeding High

3 Oluganthota 60//70 Pavement edge failures Low

4 Haputhale 60/70 No significant distress -

Table 3: Survey locations

Figure 3: Thermocouple installed in the pavement

Figure 4: Pavement distresses

(a) Bleeding at location 1 (b) Shoving and rutting at location 1

(c) Bleeding at location 2 (d) Edge cracks at location 3



Surface distresses; bleeding, shoving and rutting have

been observed in sections with soft bitumen. Surface

distresses in those sections were a result of the use of

softer bitumen in hot climatic zones. Isolated edge cracks

are load related cracks induced near the joint of old and

new pavement of road widening sections. This analysis

clearly shows that the bitumen stiffness affects surface

distresses significantly.

Analysis of binder properties

Temperature susceptibility (experimental approach)

Penetration values measured at different temperatures

in the laboratory for the 80/100 and 60/70 binders are

shown in Figure 5. It clearly shows that the binders

used in the industry are highly temperature susceptible.

The samples, which have lower penetration values at

25 oC show higher penetration values at slightly higher

temperatures. Generally, a selected bitumen is checked

for penetration at 25 oC in the penetration grading system

and the same penetration grade binder can perform

differently at service temperatures. This is one of the

major disadvantages of the penetration grade bitumen

and it has been shown in laboratory testing.

Temperature susceptibility (theoretical approach)

Calculated penetration index (PI) values for the 34 test

reports collected from the contractors are shown in

Figure 6. The penetration index should be in the range

of +1 to -1 for binders used for road constructions

(Roberts et al., 1996). According to the penetration index

computed from test reports, 50 % of samples are out of

the range, i.e. temperature susceptible.

Test report data analysis

Penetration Variation

The penetration values of 24 test reports of 60/70 grade

and 27 test reports of 80/100 grade were used for the

analysis. Figure 7 shows the penetration values of 60/70

and 80/100 binders with their allowable ranges. In the

60/70 grade, 29 % of the samples are out of the range and

33 % of the samples are at or near the boundary. In the

80/100 grade, all the samples are in the specified range,

but shows a high variation of the test results.

Softening point variation

Softening point in 18 test reports from 60/70 grade and

17 test reports in 80/100 grade were used in the analysis.

In 60/70 grade, all values were at or near the lower limit

and in 80/100 grade, 70 % of the samples were out of the

range and the rest of the samples (30 %) were at or near

the lower boundary. The softening point results of 60/70

and 80/100 grades are given in Figure 8.

Softening point and penetration

Figure 8 shows the plot of softening point and penetration

of 60/70 and 80/100 binders. The figure shows that 22 %

of samples from 60/70 grade and 73 % of samples from

80/100 grade are out of the allowable limit. But most of

the samples of 60/70 are in the boundary of the allowable

limit. The samples within the range in both cases are less

than 20 %.

Mixing and laying temperatures

The mixing and laying temperatures depend on the

viscosity and should be at a viscosity of 2.8 ± 0.3 and 1.7

± 0.2 Poises, respectively (Roberts et al., 1996). Since the

viscosity is not measured in the present binder grading

system, the mixing and laying temperatures are selected

arbitarly. Figure 9 shows the asphalt cement mixing and

laying temperatures of 87 data records collected from

different contractors. The ICTAD specification for roads

and bridges specifies the minimum mixing and laying

temperatures to be 150 oC and 135 oC, respectively. It was

found in the survey that the contractors have not selected

the appropriate mixing and compaction temperature

based on the viscosity and specification limits.

Location Name of the place Distance from Mean sea Temperature oC

Rathnapura (km) level (m) Min Max Avg

1 Pelmadulla 19 122 24 35 27

2 Allepola - Balangoda 41 600 24 35 27

3 Oluganthota - Balangoda 50 530 24 35 27

4 Haputhale 89 1431 13 25 21

Min - Minimum ; Max - Maximum ; Avg - Average

Table 4: Inventory data of selected locations



Questionnaire results

According to the questionnaire, industry people believed

that the asphalt binder grade affects pavement distresses

such as stripping, raveling, bleeding, fatigue, rutting,

shoving and corrugation. More than 50 % of the industry

experts did not agree with the current penetration grading

system, because it measures the properties of asphalt

binders only at one temperature (25 oC) and does not

measure the real bitumen properties at adverse service

temperatures.

Generally, the frequency of the asphalt binder

testing specified is one test per 100 tons and industry

does not check the properties according to the specified

Figure 6: Penetration variations of tested samples

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Figure 7: Softening point variation of tested samples of Asphalt

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Figure 8: Softening point and penetration variation of 60/70 and

80 − 100

52

54

56

So

ften

ing P

oin

tSoftening point vs Penetration (80/100 and 60/70)

60/70 Binder

80/100 Binder

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46

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50

40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120

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ften

ing P

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enin

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Penetration

Softening point vs Penetration (80/100 and 60/70)

Figure 5: Temperature susceptibility of binders

Temperature Succeptibility of 80/100 and 60/70 binders

Penetr

ation (

0.1

) m

m

Temperature oC

Figure 9: Mixing and laying temperatures

Mixing and Laying Temperatures

Tem

pera

ture

oC

Sample



Figure 10: ArcGIS map based on temperature

(a) Maximum temperature (b) Mean temperature (c) Minimum temperature

Location 1 Morning Evening

Experimental Theoretical Experimental Theoretical

Surface temperature 54.3 ° C 54.54 ° C 58.9 ° C 59.24 ° C

Pavement temperature 50.63 ° C 50.92 ° C 55.35 ° C 55.39 ° C

(20 mm below)

Location 2 Morning Evening

Experimental Theoretical Experimental Theoretical

Surface temperature 54.5 ° C 54.74 ° C 59 ° C 59.34 ° C

Pavement temperature 50.6 ° C 51.11 ° C 55.5 ° C 55.48 ° C

(20 mm below)

Table 5: Equation verification test results

frequency. It was found that, the bitumen manufactures

also check only the penetration and the softening point

of the bitumen binders and do not conduct the other tests

as given in the specification. The contractors only check

the penetration and softening point of the binders due

to lack of laboratory facilities, cost and time. Most of

the contractors check the above parameters to just meet

the specification requirements, and they do not have

an understanding of its effect on distresses in hot mix

asphalt.

More than 50 % of the people in the industry have

observed differences between the test reports of the

manufacturers and the contractors. More than 70 % have

experienced the deviation of the test results from the

allowable limits.



and penetration at 25 ºC, which relate to the properties

of bitumen at high pavement temperature, mixing

temperature and average pavement service temperature.

In this method, the temperature susceptibility can also

be checked. However, there are some disadvantages

in this method as well, i.e. the principal grading does

not accurately reflect low-temperature asphalt binder

rheology and thin film oven test residue viscosities can

vary greatly with the same viscosity grade. Therefore,

even if the asphalt binders are of the same viscosity

grade, they may behave differently after construction.

Consistency of asphalt binders varies with temperature

and neither penetration grading system nor viscosity

grading system addresses this issue. Most of the asphalt

binders used in Sri Lanka are temperature susceptible.

It is recommended to maintain the penetration index

between -1 and +1 for road asphalt to minimize the

temperature susceptibility.

The Ceylon Petroleum Corporation generally

manufactures only 60/70 grade asphalt binder but

manufactures 80/100 on the request of users. The survey

revealed that all the standard tests are conducted by the

manufacturer only on the contractor's request. Otherwise,

both manufacturers and the contractors test only the

penetration and softening points, which are directly

requested in the mix design. When the asphalt binder is

harder than the allowable range, it tends to induce cracks

in the pavements with the traffic load. At the lower

temperature, binder becomes stiff and brittle, which

induces temperature related cracks. When the binder is

softer than the required level, the binder related distresses

such as bleeding, rutting and shoving can be induced at hot

temperatures. According to the analysis of test reports,

the penetration and softening values vary widely and

may contribute to the pavement distresses obsrved in A4

roads, which used two types of binders. According to the

softening point data, most of the samples are at the lower

end or out of the range. At high elevated temperatures,

binders soften and pavement distresses such as shoving,

rutting and bleeding can occur in pavements. The current

practice of the Sri Lankan road construction industry is to

use typical temperatures indicated in specifications (broad

range) for mixing and laying temperatures. It is essential

to determine the mixing and laying temperatures since the

binder properties can vary. However, the current grading

system does not provide mixing and laying temperature

of asphalt binders.

The Superpave asphalt grading systems address

most of these issues and considers traffic loading in

binder selection. The binder grade is determined based

on the pavement temperature of an average 7 - day

Selection of binder grades and zoning with ArcGIS

Pavement temperature prediction algorithm used by

Khalil et al. (2009) can be satisfactorily used for the

pavement temperature estimation of Sri Lanka. Table 5

shows the verification results obtained at two locations in

two road sections for two different times of the day. Thus,

the validated Superpave prediction algorithms (equations

3 and 4) were used for the estimation of pavement

temperature from air temperatures, which were obtained

from the Meteorological Department for the last 20

years. Average 7 - day maximum and 1 - day minimum

of the last 20 years in the selected locations were used

to determine the Superpave binder grade for Sri Lanka.

Table 4 shows the suitable binder grades for the selected

locations in different climatic zones. Nuwara Eliya and

Bandarawela require different binder grades, which are

softer than the other binders. It is required to identify the

road sections, which are in different temperature zones to

select the suitable binder grades. The developed ArcGIS

maps (Figure 10) show the maximum, minimum and

mean temperatures of the road sections, which can be

used to determine the suitable binder grades for road

sections in Sri Lanka.

Figure 10a can be used for the methods where the

maximum temperature is critical, i.e. Indian standards

(Nagabhushana, 2009) for the penetration grading, newly

adapted viscosity grading system for India (Kandhal,

2007), and prediction of Superpave grades for different

crude oil blends (Ithnin, 2008). Figure 10b can be used

for the methods where the mean temperature is critical,

i.e. Asphalt Institute recommendations (Asphalt Institute,

1991) for the penetration grading. Figure 10c can be

used for the methods where the minimum temperature is

critical, i.e. Indian standards (Nagabhushana, 2009) for

the viscosity grading and selection of Superpave graded

binders.

CONCLUSION

The asphalt binder grading system used in Sri Lanka is

the penetration grading system. It is based on empirical

tests (such as penetration, softening point) and does not

measure any fundamental engineering parameters of

binders. The manufacturers do not provide mixing and

compaction temperatures and the contractors do not

conduct laboratory testing to determine them. Also, the

temperature susceptibility of the binder is not checked

in the present binder grading system. The viscosity

grading system is more reliable than the penetration

grading system, since the testing is carried out at three

different temperatures; viscosity at 60 ºC and 135 ºC



maximum and 1 - day minimum temperature of 20 years

of temperature data. The Superpave algorithm to convert

air temperature to pavement temperature was validated

using measured air and pavement temperatures and it was

found that equations 3 and 4 can be successfully used for

the purpose. The Superpave asphalt grading system can

be considered as the most acceptable grading system and

PG 58-16 and PG 52-10 can be introduced for hot and

cold regions of Sri Lanka, respectively.

The complexity of testing, lack of instruments and

knowledge, and high cost are some of the restraints to

adopt a completely new grading system. It is recommended

that the viscosity grading system is more reliable than the

penetration grading system. Since the testing procedure

is more or less similar to the penetration grading system,

it can be introduced to the country very easily. The

authors have identified the bitumen grade required for

Sri Lanka. AC 30 and AC 20 can be introduced for hot

climatic zones and the cold climatic zones, respectively.

Arc GIS maps can be used to identify the road sections

in the climatic zones. Table 6 shows the selected binder

grades for some selected locations.

Acknowledgement

The laboratory test data and the samples used in this

study were obtained from the R&D laboratory of the

Road Development Authority, Maga Naguma, Maga

Pvt Ltd., and some other contractors (names are not

disclosed on their request). The authors are grateful for

their assistance.

REFERENCES

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Location Zone Penetration Viscosity Superpave

Colombo 60/70 AC - 30 PG 58-16

Jaffna 60/70 AC - 30 PG 58-16

Trincomalee 60/70 AC - 30 PG 58-16

Hambantota 60/70 AC - 30 PG 58-16

Ratnapura 60/70 AC - 30 PG 58-16

Anuradhapura 60/70 AC - 30 PG 58-16

Kandy 60/70 AC - 30 PG 58-16

Bandarawela 80/100 AC - 20 PG 52-10

Nuwara Eliya 80/100 AC - 20 PG 52-10

Galle 60/70 AC - 30 PG 58-16

Puttalam 60/70 AC - 30 PG 58-16

Kurunegala 60/70 AC - 30 PG 58-16

Badulla 60/70 AC - 30 PG 58-16

Table 6: Suitable binder grades for selected locations

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